We have been attempting to elucidate how the activity of the enzyme adenylate cyclase might be altered and regulated in cultured fibroblasts. Trypsin and certain other endopeptidases have been shown to markedly enhance the adenylate cyclase activity of fibroblast membrane preparations. Brief trypsin treatment of intact fibroblasts also increases cyclase activity and results in significant enhancement of intracellular cyclic AMP levels. To assess the ability of serum and certain growth promoters to influence cyclic AMP metabolism we determined the abilty of these factors to alter the accumulation of cyclic AMP in response to added PGE1. The somatomedin-like peptide MSA is found to significantly inhibit the hormone-stimulated accumulation of cyclic AMP. We have also developed a system to study cell-free hormonal desensitization of the cyclase. When isolated crude NRK plasma membranes are incubated with isoproterenol under appropriate conditions a time and hormone-concentration dependent decrease in the isoproterenol response of the cyclase system can be demonstrated.